Preparation of solid solutions of metallic oxide mixtures by flame-spraying

ABSTRACT

A SOLID SOLUTION OF A VALVE DIOXIDE AND A PRECIOUS METAL DIOXIDE IS PREPARED IN BULK FORM INDEPENDENT OF A SUBSTRATE BY PASSING, IN FINELY-DIVIDED ATOMIZED FORM, AN ORGANIC SOLUTION OF A SALT OR ESTER OF THE VALVE METAL AND A SALT OF THE PRECIOUS METAL THROUGH AN OXIDIZING FLAME.

United States Patent 3,677,975 PREPARATION OF SOLID SOLUTIONS OFMETALLIC OXIDE MIXTURES BY FLAME- SPRAYING John E. Bennett, Painesville,and Kevin J. OLeary, Cleveland Heights, Ohio, assignors to DiamondShamrock Corporation, Cleveland, Ohio No Drawing. Filed Oct. 26, 1970,Ser. No. 84,111 Int. Cl. H01b 1/06; B05b 7/20; C22b 11/00 US. Cl.252-518 8 Claims ABSTRACT OF THE DISCLOSURE A solid solution of a valvemetal dioxide and a precious metal dioxide is prepared in bulk formindependent of a substrate by passing, in finely-divided atomized form,an organic solution of a salt or ester of the valve metal and a salt ofthe precious metal through an oxidizing flame.

BACKGROUND OF THE INVENTION A wide variety of solid solutions have longbeen known and used in the art. Recently, specifific types of solidsolutions have been developed which are particularly useful aselectrically-conducting components in electrolytic applications. Theseparticular solid solutions, composed primarily of mixtures of valvemetal and precious metal oxides, are formed when heated in contact witha conductive base as described more fully, for example, in South AfricanPat. 68/0834. Anodes prepared in accordance with this teaching exhibitproperties so superior to prior known and used dimensionally-stableanodes that they have enjoyed ever increasing and widespread commercialuse, particularly in chlor-alkali electrolytic processes. Because of theexcellent electroconductive and catalytic properties of these anodes,many attempts accordingly have been made in the art to adapt the solidsolutions on which they are based for use in other related applications.

It has been found, however, that in contrast to the teachings of theaforementioned South African patent, true solid solutions, particularlyvalve metal dioxideprecious metal dioxide solid solutions, are obtainedonly when the component metal oxides are formed in contact with aconductive base. Additionally, it has been found from practice of theaforesaid South African patent teachings, that certain electroconductivesolid solutions, e.g., a titanium dioxide-ruthenium dioxide solidsolutions can be so formed on only certain selected conductivesubstrates, preferably titanium. Attempts to form such solid solutions,i.e., polycrystalline materials wherein molecules of titanium in thecharacteristic tetragonal (rutile) titanium dioxide lattice are randomlyreplaced with molecules of ruthenium, have not been successfullyaccomplished on a substrate such as nickel. In contact with this metal,only adherent, physical mixtures of the oxides are ob tained,identifiable as separate crystalline phases. Attempts to form titaniumdioxide-ruthenium dioxide solid solutions likewise have not beensuccessfully accomplished on lead or other soft metals more susceptiblethan titanium to deterioration when exposed to elevated temperatures.

Widely known methods for preparing solid solutiontype materials in bulkform as set forth, for example, in US Patent Nos. 3.498,931 and3,514,414, have the disadvantage of requiring the use of sealed reactionvessels, vacuum, high pressure and the like and are best adapted only tosmall scale operation. A much simpler technique for preparing solidsolutions have been developed which requires no such sealed equipment orstrenuous operating conditions. This method which is described in apending US. patent application, Ser. No. 74,876, filed in the name ofJames M. Kolb, generally involves slowly heating an intimate mixture ofa finely-divided valve metal and a salt of a precious metal in anoxidizing atmosphere for a sufficient time period to convert the valvemetal and precious metal to their respective dioxides. With this methol,the desired metallic oxidation state typically is attained by heatingthe aforesaid mixture at a prescribed slow rate up to a temperature of450-600 C., which process may involve 24 hours heating time or evenlonger.

SUMMARY OF THE INVENTION We have now found that a true valve metaldioxideprecious metal dioxide solid solution may easily be obtained inbulk form and independent of a substrate by first (a) Preparing asolution of a salt or an ester of a valve metal and a salt of a preciousmetal; and then (b) Flame-spraying said solution by passing it throughan oxidizing flame maintained at a temperature sufficient to convertboth the valve metal and the precious metal essentially to theirrespective dioxides with the concurrent formation of a solid solution ofsaid meta-l dioxides.

From the flame zone, the solid solution thus formed may be passed intosuitable apparatus wherein it can be collected in particulate form forfuture use in a variety of applications typically with no furtherchemical or modifying treatment. It may easily be applied, for example,either alone or in combination with other ingredients onto a conductiveand/or supporting base.

Alternatively, of course, the solid solution may be directly sprayedonto a conductive and/ or supporting base, forming a hard, tightlybonded and durable conductive coating thereon.

The process of this invention has many distinct and commerciallyimportant advantages over all prior art methods for preparing solidsolutions of this type in bulk form independent of the metal base.First, this process is a much faster and more economical method than thealready known techniques, since the flame-spraying and subsequentproduct collection or deposition steps together may be effected in atotal time period of one several seconds. Also, it is a simple, safemethod for forming solid solutions, requiring no expensive equipmentand/or hazardous operations as pressure-sealed tubes, inert atmospheres,high vacuum, high pressure, etc. Of utmost importance, of course, is thefact that by practice of this invention, it is possible to prepare valvemetal dioxideprecious metal dioxide solid solutions which consistentlyare of a high degree of purity. Particularly, TiO Ruo solid solutions ofthis invention contain no detectable free precious metal dioxide as aseparate crystalline phase.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The solid solutions of thepresent invention are mixtures of oxides of the various valve metals andprecious metals. As used herein, the term valve metals has the standardconnotation and refers to titanium, zirconium, niobium and tantalummetals. The term precious metal is intended herein to refer primarily tothe platinum group metals: platinum, ruthenium, iridium, palladium,rhodium, and osmium.

In the practice of this invention, either a suitable ester or salt ofthe valve metal may be employed. Suitable esters are compounds,preferably liquid, which have the general formula Me(OR) wherein Me isthe valve metal; R is a hydrocarbon radical, preferably aliphatic,having from about 3 to 8 carbon atoms; and x is the valence of themetal. In the presently preferred embodiments of the invention whereintitanium is the valve metal, specific esters thereof suitably usedinclude, for example, tetraisopropyl titanate, tetrabutyl titanate, ortetra 2-ethylhexyl titanate. It should be noted, however, that any esterof a valve metal generally may be used providing said ester, if not aliquid, can be easily dissolved in the organic liquid solvating mediumwhich is employed in preparing the mixed metallic soution. Valve metalsalts suitably employed include the halides thereof, e.g. TiCl and othersalts which can be decomposed at the flamespraying temperatures utilizedherein.

The precious metals are employed typically in a powdered salt form,e.g., in the form of hydrated halides such as RuCl -2.5 H O, etc.,usually for convenience and owing to commercial availability of thesematerials. However, any decomposable form of the previous metal, e.g.,an ester thereof, can be used.

In general, the amounts of the two metallic compounds which may beemployed to form the solution subsequently flame-sprayed in the practiceof this invention depend, of course, upon the relative amounts ofprecious metal oxide which the valve metal oxide crystal lattice willincorporate into a solid solution conforming to the formula Y Z Owherein Y=the precious metal and Z=the valve metal. In the case of theRuO TiO system, the amount of precious metal incorporated generally willvary between 0.01 and 99+%, as the metallic oxide molecules are freelyinterchangeable in the lattice. Within this range, the amount ofruthenium actually employed is varied depending upon the intended use ofthe solid solution and economic considerations, use of 80% RuO beingtypical.

In carrying out the present invention, the solution of the preciousmetal salt and the valve metal compound to be flame-sprayed is preparedin an organic solvating medium such as a lower alkanol, e.g.,isopropanol or n-butanol. In practice of presently preferred embodimentsherein, the precious metal salt, as RuCl -2.5 H O usually is first mixedwith a small amount of a mineral acid, e.g., HCl, to aid its subsequentdissolution in the alcohol. The precious metal salt-acid mixture is thenmixed with the alcohol and the metal salt completely dissolves.Thereafter, the valve metal, e.g., titanium compound is dissolved in theprecious metal salt solution. After preparation, the metallic solutionmay immediately be flame-sprayed or, if necessary, may be stored forseveral hours prior to processing, with equivalent results.

The prepared metallic solution is passed as a finelydivided atomizedspray through an oxidizing flame to form the desired valve metaldioxide-precious metal dioxide solid solution. Typically, the solutionwill be propelled into the ignition zone by a carrier gas, such as air,in admixture with a fuel gas. Aspiration of the solutiongas mixtureusually provides impetus to the atomization process. However, anyflame-spraying technique now known for flame spraying, e.g., metalpowders, may be successfully employed herein.

From the flame zone, the product may be drawn by vacuum into a suitabledevice such as a cyclone separator wherein it is isolated from thegaseous phase and collected in particulate form. Alternatively, aspoined out previously herein, the solid solution may be sprayed onto aconductive and/or supporting base. In the case of heatsensitive basematerials, such a substrate is positioned at a prescribed distance fromthe flame so as not to be distorted or otherwise deleteriously affectedthereby.

In carrying out the flame-spraying operation, the temperature of theoxidizing flame is quite critical and must be maintained within therange of 900-1600 C., preferably from l000l500 'C., particularly whenforming Ti0 -RuO solid solutions. If passed through a flame which ishotter than 1600 C., the metals, especially a precious meta such asruthenium, will be converted to oxides higher than the desired dioxide.Such higher oxides are, in most instances, vaporous compounds.Conversely, if the metallic solution is aspirated through a flamemaintained at a temperature much below 900 C., the desired oxidationstate of these metals will not be attained to any significant degree, ifat all. Further, if any metal dioxides should be formed, these usuallywill exist only in separate phases and are easily identifiable as such,no true solid solution being obtained.

As the flame source, any fuel suitably may be employed in an oxygenatmosphere, e.g., in air, providing the heat generated by thefuel-oxidant flame can be controlled within the above-described criticaltemperature range. To date, mixtures of either natural gas or hydrogenwith air have been easily regulated to provide oxidizing flames of thedesired temperature. Under the same operating conditions, however, nosimilar acetylene mixture, whether rich or deficient in air, has beenfound satisfactory up to the present time. Flame temperatures canconveniently be measured herein by means of an optical pyrometer.

In carrying out flame-spraying operations, the temperature of theoxidizing flame is regulated within the proper range by adjusting theflow rates of the air and fuel streams entering the burner of theflame-spraying apparatus. Typically, when employing hydrogen as thefuel, a flow rate of about 5 to 14 cubic feet of air/hour for each cubicfoot/hour of hydrogen will provide eflicient oxidant-fuel mixtures.

It is further to be noted that regulation of the oxidantfuel flow rateslikewise serves to provide, in conjunction with the burner head, anoxidizing flame of the proper dimensions, e.g., height, therebypreventing excessive exposure of the sprayed solution to intense heatand conversion of the metal components to undesirable oxides. Dependingupon the air-fuel mixtures employed in practice of this invention,selection of burner heads with the proper width can be easily determinedwith minimum experimentation. When employing air-hydrogen mixtureswithin the aforementioned flow rates, use of a burner head about 4inches wide has been found particularly suitable.

The valve metal oxide-precious metal oxide solid solutions of thisinvention obtained either as powders or as coherent coatings, areelectrically conductive materials. These products consistently arehighly pure products, as indicated by X-ray analysis of theircrystalline structure. In presently preferred embodiments of thisinvention, wherein titanium and ruthenium respectively are theparticular valve and precious metals employed, X-ray diffractionpatterns of the solid solutions obtained show these to be essentiallyrutile form solid solutions of titanium and ruthenium dioxides. Noevidence is found indicating the presence of any free precious metaldioxide in these products.

The valve metal oxide-precious metal oxide solid solution products ofthis invention are useful in a variety of applications. For example, ithas been found that electrical resistors of predictable degrees ofresistivity are readily formed by varying the relative amounts of valveand precious metal components, the relationship between the mole percentof precious metal present and the electrical conductivity being areproducible function. This property also makes solid solutions of thepresent invention useful in the manufacture of capacitors and otherelectronic components.

The solid solutions have also proven useful as nonspecific oxidationcatalysts for gaseous phase reactions. The catalytic activity of thesolid solutions appears extremely high and use of these materials asexhaust emission control catalysts is suggested.

In addition, owing to their excellent electroconductivity andelectrocatalyitc activity, the solid solutions are useful in a number ofelectrochemical applications, especially in view of the fact that theymay easily be applied, e.g.,

by spraying, onto a variety of supporting and conducting substrates,which was not true in previous electrochemical applications where theiruse on a substrate of the parent valve metal was usually dictated. Usein fuel cell applications is particularly promising.

In order that those skilled in the art may more readily understand thepresent invention and the preferred methods by which it may be carriedout, the following specific examples are offered. In these exampleswhere proportions of ingredients used may be expressed in parts, suchproportions are by weight.

EXAMPLE 1 A solution containing for each part of RuCl -XH O (0.4 part Rumetal), 6.2 ml. n-butyl alcohol, 3.0 ml. tetrabutyl orthotitanate, and0.4 ml. 36% HCl, is aspirated into an air stream of 32.3 cubic feet perhour. This stream is then combined with 2.66 cubic feet per hour ofhydrogen and ignited at a burner head 4 inches wide to produce a dullorange flame. The products of the flame are passed through a cyclonewhere the solid oxides produced are separated from the gases present.

Product thus collected is an electrically conductive black powder. It isshown by X-ray analysis to be a substantially pure rutile form solidsolution of titanium and ruthenium dioxides, providing only one X-rayreflection from each crystallographic plane. In contrast, mixtures ofthese dioxides in free form provide interplanar spacings correspondingto each separate crystalline phase. Interplanar spacing of the 110crystal plane for the product is 3.20 A. as compared to 3.18 A. for freeruthenium dioxide and to 3.245 A. for free titanium dioxide.

EXAMPLE 2 A solution prepared as in Example 1 is again aspirated into anair stream of 32.3 cubic feet per hour. This stream is then combinedwith 5.07 cubic feet per hour of hydrogen and ignited at a burner headto produce a light organe flame.

Product collected as in Example 1 is again an electrically conductiveblack powder indicated by X-ray analysis to be a substantially purerutile fonm solid solution of titanium and ruthenium dioxides.Interplanar spacing of the 110 crystal plane for this material is 3.22A.

In contrast to the foregoing examples, flame-spraying of identicalmetallic solutions under similar conditions using various air-acetylenemixtures as the oxidant-fuel flame source did not produce the desiredsolid solutions. The products thus obtained were nonconductive powdersgrayish in color. These products were found, by X-ray analysis, to bepredominantly mixtures of anatase and rutile titanium dioxide. Noruthenium dioxide was found to be present, even in separate crystallinephase.

While the invention has been described with reference to certainpreferred embodiments and techniques, it is not to be so limited sincechanges and alterations may be made therein which are fully within theintended scope of the appended claims.

We claim:

1. A method for preparing a valve metal dioxide-precious metal dioxidesolid solution in bulk form independent of a substrate which comprises:

(a) preparing a solution of an ester or a salt of a valve metal and asalt of a precious metal;

(b) flame-spraying said solution by passing it through an oxidizingflame maintained at a temperature sufficient to convert both the valvemetal and the precious metal essentially to their respective dioxideswith the concurrent formation of a solid solution of said metaldioxides.

2. The method of claim 1 wherein the valve metal is titanium and theprecious metal is ruthenium.

3. The method of claim 2 wherein the ruthenium metal is present in thesolid solution in the range of 20-80%, on an Ru0 basis.

4. The method of claim 1 wherein the solid solution of the valve metaland precious metal dioxides upon emergence from the flame zone iscollected in particulate form.

5. The method of claim 1 wherein the solid solution of the valve metaland precious metal dioxides is sprayed from the flame zone onto asupporting base.

6. The method of claim 5 wherein the supporting base is conductive.

7. The method of claim 1 wherein the oxidizing flame is maintained at atemperature ranging from 900 to 1600 C.

8. The method of claim 1 wherein the oxidizing flame is supplied by anair-hydrogen mixture introduced into the ignition zone at a flow rateratio of 5-14 cubic feet of air/hour for each cubic foot/hour ofhydrogen.

References Cited UNITED STATES PATENTS 3,305,349 2/ 1967 Bovarnick et al-.5 AA 3,172,753 3/1965 Walsh 75.5 AC 3,062,638 11/ 1962 Culbertson etal. 75-.5 BB

DOUGLAS J. DRUMMOND, Primary Examiner US. Cl. X.R.

252--520; 117221, 105.2; 75-0.5 B, 0.5 BB

